china mobile cran white paper

C-RAN

The Road Towards Green RAN

White Paper

Version 2.5 (Oct, 2011)

China Mobile Research Institute

China Mobile Research Institute

i

Table of Contents C-RAN ............................................................................................................................................... i

The Road Towards Green RAN ..................................................................................................... i

1 Introduction ............................................................................................................................ 1

1.1 Background ......................................................................................................................... 1

1.2 Vision of C-RAN .................................................................................................................. 1

1.3 Objectives of this White Paper ....................................................................................... 2

1.4 Status of this White Paper ............................................................................................... 2

2 Challenges of Today’s RAN ............................................................................................... 3

2.1 Large Number of BS and Associated High Power Consumption .............................. 3

2.2 Rapid Increasing CAPEX/OPEX of RAN.......................................................................... 4

2.3 Explosive Network Capacity Need with Falling ARPUs............................................... 6

2.4 Dynamic mobile network load and low BS utilization rate ....................................... 7

2.5 Growing Internet Service Pressure on Operator‟s Core Network............................ 7

3 Architecture of C-RAN ......................................................................................................... 9

3.1 Advantages of C-RAN ..................................................................................................... 11

3.2 Technical Challenges of C-RAN ..................................................................................... 12

4 Technology Trends and Feasibility Analysis ...................................................................... 13

4.1 Wireless Signal Transmission on Optical Network.................................................... 13

4.2 Dynamic Radio Resource Allocation and Cooperative Transmission/Reception . 20

4.3 Large Scale Baseband Pool and Its Interconnection ........................................................... 22

4.4 Open Platform Based Base Station Virtualization ................................................................ 24

4.5 Distributed Service Network ................................................................................................. 27

5 Evolution Path ...................................................................................................................... 28

5.1 C-RAN Centralized Base Station Deployment ........................................................... 28

5.2 Multi-standard SDR and Joint Signal Processing ...................................................... 28

5.3 Virtual BS on Real-time Cloud Infrastructure ............................................................ 28

6 Recent Progress .................................................................................................................. 30

6.1 TD-SCDMA and GSM Field Trial .................................................................................... 30

6.2 Large Scale Baseband Pool Equipment Development ............................................. 35

6.3 C-RAN prototype based on General Purpose Processor ................................................... 37

7 Conclusions ........................................................................................................................... 39

8 Acknowledgement .............................................................................................................. 40

9 Terms and Definitions ....................................................................................................... 41

10 Reference ............................................................................................................................ 43

Cover is for

position only

China Mobile Research Institute 1

1 Introduction

1.1 Background

Today‟s mobile operators are facing a strong competition environment. The cost to build,

operate and upgrade the Radio Access Network (RAN) is becoming more and more expensive

while the revenue is not growing at the same rate. The mobile internet traffic is surging, while

the ARPU is flat or even decreasing slowly, which impacts the ability to build out the networks

and offer services in a timely fashion.. To maintain profitability and growth, mobile operators

must find solutions to reduce cost as well as to provide better services to the customers.

On the other hand, the proliferation of mobile broadband internet also presents a unique

opportunity for developing an evolved network architecture that will enable new applications

and services, and become more energy efficient.

The RAN is the most important asset for mobile operators to provide high data rate, high

quality, and 24x7 services to mobile users. Traditional RAN architecture has the following

characteristics: first, each Base Station (BS) only connects to a fixed number of sector

antennas that cover a small area and only handle transmission/reception signals in its coverage

area; second, the system capacity is limited by interference, making it difficult to improve

spectrum capacity; and last but not least, BSs are built on proprietary platforms as a vertical

solution. These characteristics have resulted in many challenges. For example, the large

number of BSs requires corresponding initial investment, site support, site rental and

management support. Building more BS sites means increasing CAPEX and OPEX. Usually, BS‟s

utilization rate is low because the average network load is usually far lower than that in peak

load; while the BS‟ processing power can‟t be shared with other BSs. Isolated BSs prove costly

and difficult to improve spectrum capacity. Lastly, a proprietary platform means mobile

operators must manage multiple none-compatible platforms if service providers want to

purchase systems from multiple vendors. Causing operators to have more complex and costly

plan for network expansion and upgrading. To meet the fast increasing data services, mobile

operators need to upgrade their network frequently and operate multiple-standard network,

including GSM, WCDMA/TD-SCDMA and LTE. However, the proprietary platform means mobile

operators lack the flexibility in network upgrade, or the ability to add services beyond simple

upgrades.

In summary, traditional RAN will become far too expensive for mobile operators to keep

competitive in the future mobile internet world. It lacks the efficiency to support sophisticated

centralized interference management required by future heterogeneous networks, the flexibility

to migrate services to network edge for innovative applications and the ability to generate new

revenue from revenue from new services. Mobile operators are faced with the challenge of

architecting radio network that enable flexibility. In the following sections, we will explore ways

to address these challenges.

1.2 Vision of C-RAN

The future RAN should provide mobile broadband Internet access to wireless customers with

low bit-cost, high spectral and energy efficiency. The RAN should meet the following

requirements:

Reduced cost (CAPEX and OPEX)

Lower energy consumption


High spectral efficiency

Based on open platform, support multiple standards, and smooth evolution

Provide a platform for additional revenue generating services.

Centralized base-band pool processing, Co-operative radio with distributed antenna equipped

by Remote Ratio Head (RRH) and real-time Cloud infrastructures RAN (C-RAN) can address the

challenges the operators are faced with and meet the requirements. Centralized signal

processing greatly reduces the number of site‟s equipment room needed to cover the same

areas; Co-operative radio with distributed antenna equipped by Remote Radio Head (RRH)

provides higher spectrum efficiency; real-time Cloud infrastructure based on open platform and

BS virtualization enables processing aggregation and dynamic allocation, reducing the power

consumption and increasing the infrastructure utilization rate. These novel technologies provide

an innovative approach to enabling the operators to not only meet the requirements but

advance the network to provide coverage, new services, and lower support costs.

C-RAN is not a replacement for 3G/B3G standards, only an alternative approach to current

delivery. From a long term perspective, C-RAN provides low cost and high performance green

network architecture to operators. In turn operators are able to deliver rich wireless services in

a cost-effective manner for all concerned.

C-RAN is not the only RAN deployment solution that will replace all today‟s macro cell station,

micro cell station, pico cell station, indoor coverage system, and repeaters. Different

deployment solutions have their respective advantages and disadvantages and are suitable for

particular deployment scenarios. C-RAN is targeting to be applicable to most typical RAN

deployment scenarios, like macro cell, micro cell, pico cell and indoor coverage. In addition,

other RAN deployment solution can serve as complementary deployment of C-RAN for certain

case.

1.3 Objectives of this White Paper

The objective of this white paper is to present China Mobile‟s vision of C-RAN and provide a

research framework by identifying the technical challenges of C-RAN architecture. We would

like to invite both industry and academic research institutes to join the research to guide the

vision into reality in the near future.

1.4 Status of this White Paper

This document version 2.5 is a revised version of version 2.0 released in December 2010. It is

not yet fully complete and there may still be some inconsistencies. However, it is considered to

be useful for distribution at this stage. It is expected that new research challenges might be

added in future versions. Comments and contributions to improve the quality of this white

paper are welcome.


2 Challenges of Today’s RAN

2.1 Large Number of BS and Associated High Power Consumption

As operators constantly introduce new air interface and increase the number of base stations to

offer broadband wireless services, the power consumption gets a dramatic rise. For example: in

the past 5 years, China Mobile has almost doubled its number of BS, to provide better network

coverage and capacity. As a result, the total power consumption has also doubled. The higher

power consumption is translated directly to the higher OPEX and a significant environmental

impact, both of which are now increasingly unacceptable.

The following figure 1 shows the components of the power consumption of China Mobile. It

shows the majority of power consumption is from BS in the radio access network. Inside the BS,

only half of the power is used by the RAN equipment; while the other half is consumed by air

condition and other facilitate equipments.

Obviously, the best way to save energy and decrease carbon-dioxide emissions is to decrease

the number of BS. However, for traditional RAN, this will result in worse network coverage and

lower capacity. Therefore, operators are seeking new technologies to reduce energy

consumption without reducing the network coverage and capacity. Today, there are quite a

number of „amendment‟ technologies that helps reduce BS‟ power consumption, such as the

software solutions which save power through turning off selected carriers on idle hours like

midnight, the green energy solutions which offer solar, wind and other renewable energy for

base station‟s power supply according to local natural conditions, and the energy-saving air

conditioning technology which combined with the local climate and environment characteristics,

reduce the energy consumption of the air conditioning equipment, etc. However, these

technologies are supplementary methods and cannot address the fundamental problems of

power consumption with the number of increasing BS.

In the long run, mobile operators must plan for energy efficiency from the radio access network

architecture planning. A change in infrastructure is the key to resolve the power consumption

challenge of radio access network. Centralized BS would reduce the number of BS equipment

rooms, reduce the A/C need, and use resource sharing mechanisms to improve the BS

utilization rate efficiency under dynamic network load.

Channel, 6%

Transmission,

15%

Management

office, 7%

Cell site, 72%

Major

Equipment,

51%Air

Conditioners,

46%

Other Support

Equipment,

3%

Fig.1 Power Consumption of Base Station


2.2 Rapid Increasing CAPEX/OPEX of RAN

Over recent years, mobile data consumption has experienced a record growth among the

world‟s operators as subscribers use more smart phones and mobile devices, like tablets. To

satisfy this consumer usage growth, mobile operators must significantly increase their network

capacity to provide mobile broadband to the masses. However, in an intensifying competitive

marketplace, high saturation levels, rapid technological changes and declining voice revenue,

operators are challenged with deployment of traditional BS as the cost is high, the return is not

high enough. Average Revenue Per User (ARPU) are all affecting mobile operators‟ profitability.

They become more and more cautious about the Total Cost of Ownership (TCO) of their

network in order to remain profitable and competitive.

Fig. 2: Increasing CAPEX of 3G Network Construction and Evolution

Analysis of the TCO

The TCO including the CAPEX and the OPEX results from the network construction and

operation. The CAPEX is mainly associated with network infrastructure build, while OPEX is

mainly associated with network operation and management.

In general, up to 80% CAPEX of a mobile operator is spent on the RAN. This means that most

of the CAPEX is related to building up cell sites for the RAN. The historical CAPEX expenditure of

2007-2012 forest are shown in Fig.2. Because 3G/B3G signals （deployed frequency 2GHz have

higher path loss and penetration loss than 2G signals (deployed frequency 900MHz), multiple

cell sites are needed for the similar level of 2G coverage. Thus, the dramatic increase was

found in the CAPEX when building a 3G network.

The CAPEX is mainly spent at the stage of cell site constructions and consists of purchase and

construction expenditures. Purchase expenditures include the purchases of BS and

supplementary equipments, such as power and air conditioning equipments etc. Construction

expenditures include network planning, site acquisition, civil works and so on. As shown is Fig.3,

it is noticeable that the cost of major wireless equipments makes up only 35% of CAPEX, while

the cost of the site acquisition, civil works, and equipment installation is more than 50% of the

total cost. Essentially, this means that more than half of CAPEX is not spent on productive

wireless functionality. Therefore, ways to reduce the cost of the supplementary equipment and

the expenditure on site installation and deployment is important to lower the CAPEX of mobile

operators.


Fig. 3: CAPEX and OPEX Analysis of Cell Site

OPEX in network operation and the maintenance stage play a significant part in the TCO.

Operational expenditure includes the expense of site rental, transmission network rental,

operation /maintenance and bills from the power supplier. Given a 7-year depreciation period of

BS equipment, as shown in Fig.4, an analysis of the TCO shows that OPEX accounts for over 60%

of the TCO, while the CAPEX only accounts for about 40% of the TCO. The OPEX is a key factor

that must be considered by operators in building the future RAN.

The most effective way to reduce TCO is to decrease the number of sites. This will bring down

the cost for the construction of the major equipment; and will minimize the expenditure on the

installation and rental of the equipment incurred by their occupied space. Fewer sites means

the corresponding cost of supplementary equipment will also be saved. This can significantly

decrease the operators‟ CAPEX and OPEX, but results in poorer network coverage and user

experience in the traditional RAN. Therefore, a more cost-effective way must be found to

minimize the non-productive part of the TCO while simultaneously maintaining good network

coverage.

Fig. 4 TCO Analysis of Cell Site

Multi-standard environment

It is understood that the large number of legacy terminals, 2G, 3G, and B3G infrastructure will

coexist for a very long time to meet consumers‟ demand. Most of the major mobile operators

worldwide will thus have to use two or three networks (Table 1) [1]. In the new economic

climate, operators must find ways to control CAPEX and OPEX while growing their businesses.

The base station occupies the largest part of infrastructure investment in a mobile network.

Multi-mode base station is expected as a cost efficient way for operators to alleviate the cost of

network construction and O&M. In addition, sharing of hardware resources in a multi-mode

base station is the key approach to lower cost.


Table 1. Multi-Network Operation of Major Mobile Service Providers

Cellular Technologies Vodafone China Mobile

France Telecom

T-Mobile

Verizon SK Telecom

Telstra China Unicom

TD-SCDMA √

WCDMA √ √ √ √ √ √

CDMA One & 2000 &

EVDO √ √

GSM GPRS EDGE √ √ √ √ √ √

LTE √ √ √ √

2.3 Explosive Network Capacity Need with Falling ARPUs

Data rate of mobile broadband network grows significantly with the introduction of air-interface

standards such as 3G and B3G; this in turn speeds up end user‟s mobile data consumption.

Some forecasts indicated the number of people who access mobile broadband will triple in next

several years, after LTE and LTE-A are deployed. These findings reflect the fact that the

increasing bandwidth of wireless broadband triggers the increase in mobile traffic, because the

mobile users can use a variety of high-bandwidth services, such as video-based applications.

This new trend will become a serious challenge to future RAN.

Based on the forecast data [2], global mobile traffic increases 66-fold with a compound annual

growth rate (CAGR) of 131% between 2008 and 2013. The similar trend is observed in current

CMCC network. On the contrary, the peak data rate from UMTS to LTE-A only increases with a

CAGR of 55%. Clearly, as shown in Fig.5, there is a large gap between the CAGR of new air

interface and the CAGR of customer‟s need. In order to fill this gap, new infrastructure

technologies need to be developed to further improve the performance of LTE/LTE-A.

Fig. 5 Mobile Broadband Data-rates/Traffic Growth

On the other hand, the revenue of mobile operators is not increasing at the same pace as the

network capacity they provide. Mobile operators‟ voice volumes are steadily increasing and the

data volume grows quickly, but revenues are not and ARPUs are even falling in some case. In

order to face the slow growth in revenue, operators are forced to constantly hold down costs –

notably operating costs. That means mobile operators must find a low cost, high-capacity

access network with novel techniques to meet the growth of mobile data traffic while keeping a

healthy, profitable growth.


2.4 Dynamic mobile network load and low BS utilization rate

One characteristic of the mobile network is that subscribers are frequently moving from one

place to another. From data based on real operation network, we noticed that the movement of

subscribers shows a very strong time-geometry pattern. Around the beginning of working time,

a large number of subscribers move from residential areas to central office areas for work;

when the work hour ends, subscribers move back to their homes. Consequently, the network

load moves in the mobile network with a similar pattern，so called "tidal effect". As shown in

Fig.6, during working hours, the core office area‟s Base Stations are the busiest; in the non-

work hours, the residential or entertainment area‟s Base Stations are the busiest.

Fig. 6 Mobile Network Load in Daytime

Each Base Station‟s processing capability today can only be used by the active users in its cell

range, causing idle BS in some areas/times and oversubscribed BS in other areas. When

subscribers are moving to other areas, the Base Station just stays in idle with a large of its

processing power wasted. Because operators must provide 7x24 coverage, these idle Base

Stations consume almost the same level of energy as they do in busy hours. Even worse, the

Base Stations are often dimensioned to be able to handle a maximum number of active

subscribers in busy hours, thus they are designed to have much more capacity than the

average needed, which means that most of the processing capacity is wasted in non-busy time.

Sharing the processing and thus the power between different cell areas is a way to utilize these

BS more effectively.

2.5 Growing Internet Service Pressure on Operator’s Core Network

With the hyper-growth of smart phones as well as emerging 3G embedded Internet Notebook,

the mobile internet traffic has been grown exponentially in the last few years and will continue

to grow more than 66x in the next 5-6 years. However because of increasingly competition

between mobile operators, the projected revenue growth will be much lower than the traffic

growth. There will be a huge gap between the cost associated with this mobile internet traffic

and the revenue generated, let alone the mobile operators needing to spend billions of dollars

to upgrade their back-haul and core network to keep up with the growing pace. This is a huge

common challenge to all the mobile operators in the wireless industry.

The exponential growth of mobile broadband data puts pressure on operators‟ existing packet

core elements such as SGSNs and GGSNs, increasing mobile Internet delivery cost and

challenging the flat-rate data service models. The majority of this traffic is either Internet

bound or sourced from the Internet. Catering to this exponential growth in mobile Internet

traffic by using traditional 3G deployment models, the older 3G platform is resulting in huge


CAPEX and OPEX cost while adding little benefit to the ARPU. Additional issues are the

continuous CAPEX spending on older SGSNs & GGSNs, the higher Internet distribution cost, the

congestion on backhaul and the congestion on limited shared capacity of base stations.

Therefore, offloading the Internet traffic, as close to the base stations as possible, can be an

effective way to reduce the mobile Internet delivery cost.

Fig. 7 Wireless traffic on a commercial 3G

Meanwhile it is interesting to understand how people are using today‟s mobile internet. A recent

research paper [3] published by one major TEM may give us a glimpse of the most popular

mobile applications. It is surprising to see that people are gradually using mobile internet just

like they use the fixed broadband network. Content services which include content delivered

through web and P2P are actually dominating the network traffic. Fig.7 is an example of

wireless traffic on a commercial 3G operator. Considering this usage pattern, do we have better

choice than just blindly spending billions of dollars to upgrade back-haul and the core network?


3 Architecture of C-RAN

We believe Centralized processing, Cooperative radio, Cloud, and Clean (Green) infrastructure

Radio Access Network (C-RAN) is the answer to solve the challenges mentioned above. It‟s a

natural evolution of the distributed BTS, which is composed of the baseband Unit (BBU) and

remote radio head (RRH). According to the different function splitting between BBU and RRH,

there are two kinds of C-RAN solutions: one is called „full centralization‟, where baseband (i.e.

layer 1) and the layer 2, layer 3 BTS functions are located in BBU; the other is called „partial

centralization‟, where the RRH integrates not only the radio function but also the baseband

function, while all other higher layer functions are still located in BBU. For the solution 2,

although the BBU doesn‟t include the baseband function, it is still called BBU for the simplicity.

The different function partition method is shown in Fig.8.

Fig. 8 Different Separation Method of BTS Functions

Based on these two different function splitting methods, there are two C-RAN architectures.

Both of them are composed of three main parts: first, the distributed radio units which can be

referred to as Remote Radio Heads (RRHs) plus antennas which are located at the remote site;

second, the high bandwidth low-latency optical transport network which connect the RRHs and

BBU pool; and third, the BBU composed of high-performance programmable processors and

real-time virtualization technology.

RRH

RRH

RRH

RRH

RRH

RRH

RRH

Virtual BS Pool

L1/L2/L3/O&M L1/L2/L3/O&M L1/L2/L3/O&M

Fiber

Fig. 9 C-RAN Architecture 1: Fully Centralized Solution

GPS

Main Control & Clock

Core net-work

Base-band

process-ing

Transmitter/Receiver

PA&

LNA… … ……

Antenna

DigitalIF

Solution 1Solution 2

BBU RRU


RRH/L1

RRH/L1

RRH/L1

RRH/L1

RRH/L1

RRH/L1

RRH/L1

Virtual BS Pool

L2/L3/O&M L2/L3/O&M L2/L3/O&M

Fiber or

Microwave

Fig. 10 C-RAN Architecture 2: Partial Centralized Solution

The „fully centralized‟ C-RAN architecture, as shown in figure 9, has the advantages of easy

upgrading and network capacity expansion; it also has better capability for supporting multi-

standard operation, maximum resource sharing, and it‟s more convenient towards support of

multi-cell collaborative signal processing. Its major disadvantage is the high bandwidth

requirement between the BBU and to carry the baseband I/Q signal. In the extreme case, a TD-

LTE 8 antenna with 20MHz bandwidth will need a 10Gpbs transmission rate.

The „partial centralized‟ C-RAN architecture, as shown in figure 10, has the advantage of

requiring much lower transmission bandwidth between BBU and RRH, by separating the

baseband processing from BBU and integrating it into RRH. Compared with the „full centralized‟

one, the BBU-RRH connection only need to carry demodulated data, which is only 1/20~1/50 of

the original baseband I/Q sample data. However, it also has its own shortcomings. Because the

baseband processing is integrated into RRH, it has less flexibility in upgrading, and less

convenience for multi-cell collaborative signal processing.

With either one of these C-RAN architectures, mobile operators can quickly deploy and make

upgrades to their network. The operator only needs to install new RRHs and connect them to

the BBU pool to expand the network coverage or split the cell to improve capacity. If the

network load grows, the operator only needs to upgrade the BBU pool‟s HW to accommodate

the increased processing capacity. Moreover, the „fully centralized solution‟, in combination with

open platform and general purpose processors, will provide an easy way to develop and deploy

software defined radio (SDR) which enables upgrading of air interface standards by software

only, and makes it easier to upgrade RAN and support multi-standard operation.

Different from traditional distributed BS architecture, C-RAN breaks up the static relationship

between RRHs and BBUs. Each RRH does not belong to any specific physical BBU. The radio

signals from /to a particular RRH can be processed by a virtual BS, which is part of the

processing capacity allocated from the physical BBU pool by the real-time virtualization

technology. The adoption of virtualization technology will maximize the flexibility in the C-RAN

system.


Both solutions described above are under development and evaluation. They could be properly

deployed in different networks depending on the situation of the network. The following

discussion will focus on the „Fully Centralized Solution‟.

3.1 Advantages of C-RAN

The benefits of the C-RAN architecture are listed as follows:

Energy Efficient/Green Infrastructure

C-RAN is an eco-friendly infrastructure. Firstly, with centralized processing of the C-RAN

architecture, the number of BS sites can be reduced several folds. Thus the air conditioning

and other site support equipment‟s power consumption can be largely reduced. Secondly,

the distance from the RRHs to the UEs can be decreased since the cooperative radio

technology can reduce the interference among RRHs and allow a higher density of RRHs.

Smaller cells with lower transmission power can be deployed while the network coverage

quality is not affected. The energy used for signal transmission will be reduced, which is

especially helpful for the reduction of power consumption in the RAN and extend the UE

battery stand-by time. Lastly, because the BBU pool is a shared resource among a large

number of virtual BS, it means a much higher utilization rate of processing resources and

lower power consumption can be achieved. When a virtual BS is idle at night and most of

the processing power is not needed, they can be selectively turned off (or be taken to a

lower power state) without affecting the 7x24 service commitment.

Cost-saving on CAPEX &OPEX

Because the BBUs and site support equipment are aggregated in a few big rooms, it is much

easier for centralized management and operation, saving a lot of the O&M cost associated

with the large number of BS sites in a traditional RAN network. Secondly, although the

number of RRHs may not be reduced in a C-RAN architecture its functionality is simpler, size

and power consumption are both reduced and they can sit on poles with minimum site

support and management. The RRH only requires the installation of the auxiliary antenna

feeder systems, enabling operators to speed up the network construction to gain a first-

mover advantage. Thus, operators can get large cost saving on site rental and O&M.

Capacity Improvement

In C-RAN, virtual BS‟s can work together in a large physical BBU pool and they can easily

share the signaling, traffic data and channel state information (CSI) of active UE‟s in the

system. It is much easier to implement joint processing & scheduling to mitigate inter-cell

interference (ICI) and improve spectral efficiency. For example, cooperative multi-point

processing technology (CoMP in LTE-Advanced), can easily be implemented under the C-

RAN infrastructure.

Adaptability to Non-uniform Traffic

C-RAN is also suitable for non-uniformly distributed traffic due to the load-balancing

capability in the distributed BBU pool. Though the serving RRH changes dynamically

according to the movement of UEs, the serving BBU is still in the same BBU pool. As the

coverage of a BBU pool is larger than the traditional BS, non-uniformly distributed traffic

generated from UEs can be distributed in a virtual BS which sits in the same BBU pool.

Smart Internet Traffic Offload

Through enabling the smart breakout technology in C-RAN, the growing internet traffic from

smart phones and other portable devices, can be offloaded from the core network of

operators. The benefits are as follows: reduced back-haul traffic and cost; reduced core

network traffic and gateway upgrade cost; reduced latency to the users; differentiating


service delivery quality for various applications. The service overlapping the core network

also supplies a better experience to users.

3.2 Technical Challenges of C-RAN

The centralized C-RAN brings lots of benefits in cost, capacity and flexibility over traditional

RAN, however, it also has some technical challenges that must be solved before deployment by

mobile operators.

Radio over Low Cost Optical Network

In C-RAN architecture 1, the optical fiber between BBU pool and RRHs has to carry a large

amount of baseband sampling data in real time. Due to the wideband requirement of LTE/LTE-A

system and multi-antenna technology, the bandwidth of optical transport link to transmit

multiple RRHs‟ baseband sampling data is 10 gigabit level with strict requirements of

transportation latency and latency jitter.

Advanced Cooperative Transmission/Reception

Joint processing is the key to achieve higher system spectrum efficiency. To mitigate

interference of the cellular system, multi-point processing algorithms that can make use of

special channel information and harness the cooperation among multiple antennas at different

physical sites should be developed. Joint scheduling of radio resources is also necessary to

reduce interference and increase capacity.

To support the above Cooperative Multi-Point Joint processing algorithms, both end-user data

and UL/DL channel information needs to be shared among virtual BSs. The interface between

virtual BSs to carry this information should support high bandwidth and low latency to ensure

real time cooperative processing. The information exchanged in this interface includes one or

more of the following types: end-user data package, UE channel feedback information, and

virtual BS‟s scheduling information. Therefore, the design of this interface must meet the real-

time joint processing requirement with low backhaul transportation delay and overhead.

Baseband Pool Interconnection

The C-RAN architecture centralizes a large number of BBUs within one physical location, thus

its security is crucial to the whole network. To achieve high reliability in case of unit failure, in

order to recover from error, and to allow flexible resource allocation of BBU, there must be a

high bandwidth, low latency, low cost switch network with flexible, extensible topology that

interconnects the BBUs in the pool. Through this switch network, the digital baseband signal

from any RRH can be routed to any BBU in the pool for processing. Thus, any individual BBU

failure won‟t affect the functionality of the system.

Base Station Virtualization Technology

After the baseband processing units have been put in a centralized pool, it is essential to design

virtualization technologies to distribute/group the processing units into virtual BS entities. The

major challenges of virtualization are: real-time processing algorithm implementation,

virtualization of the baseband processing pool, and dynamic processing capacity allocation to

deal with the dynamic cell load in system.

Service on Edge

Unlike service in a data center, distributing services on the edge of the RAN has its unique

challenges. In the following research framework part, we try to summarize these challenges

into the following three categories: services on the edge‟s integration with the RAN, intelligence

of DSN, and the deployment and management of distributed service.


4 Technology Trends and Feasibility Analysis

In order to solve the technical challenges of C-RAN architecture, based on current technical

conditions and future development trends, we suggest to do further research in the following

areas. The purpose is to solve the low cost high bandwidth wireless signal transmission problem

based on an optical network, dynamic resource allocation and collaborative radio technology. It

also comprehends the large scale BBU pool and associated interconnection problem, virtualized

BS based on open platforms and distributed service network solutions. The following is a

detailed analysis and discussion of these challenges.

4.1 Wireless Signal Transmission on Optical Network

The C-RAN architecture, which consists of the distributed RRH and BBU, means that need to

transport untreated wireless signals between BBU and RRH. The BBU-RRH connectivity

requirements pose challenges to the optical transmission speed and capacity. Usually, optical

fiber transmission must be used to carry the BBU-RRH signal to meet the strict bandwidth and

delay requirements.

BBU-RRH Bandwidth Requirement

Air interface is upgrading rapidly, new technologies like multiple antenna technology (2 ~ 8

antenna in every sector), wide bandwidth (10 MHz ~ 20 MHz every carrier) has been widely

adopted in LTE/LTE-A, thus the bandwidth of CPRI/Ir/OBRI (Open BBU-RRH Interface) link

bandwidth is much higher than the 2G and 3G era. In general, the system bandwidth, the

MIMO antenna configuration and the RRH concatenation levels are the main factors which have

an impact on the OBRI bandwidth requirement. For example, the bandwidth for 200 kHz GSM

systems with 2Tx/2Rx antennas and 4xsampling rate is up to 25.6Mbps. The bandwidth for

1.6MHz TD-SCDMA systems with 8Tx/8Rx antennas and 4 times sampling rate is up to

330Mbps. The transmission of this level of bandwidth on fiber link is matured and economic.

However, with the introducing of multi-hop RRH and high orders MIMO supporting 8Tx/8Rx

antenna configuration, the wireless baseband signal bandwidth between BBU-RRH would rise to

dozens of Gbps. Therefore, exploring different transport schemes for the BBU-RRH wireless

baseband signal is very important for C-RAN.

Transportation Latency, Jitter and Measurement Requirements

There are also strict requirements in terms of latency, jitter and measurement. In CPRI/Ir/OBRI

transmission latency, due to the strict requirements of LTE/LTE-A physical layer delay

processing also improve the baseband wireless signal transmission delay jitter and

requirements indirectly. Not including the transmission medium between the round-trip time

(i.e., regardless of delays caused by the cable length), for the user plane data (IQ data) on the

CPRI/Ir/OBRI links, the overall link round-trip delay may not exceed 5μs. The OBRI interface

requires periodic measurement of each link or multi-hop cable length. In terms of calibration,

the accuracy of round trip latency of each link or hop should satisfy ±16.276ns [4].

System Reliability

For the reliability of the system, because the traditional optical transmission networks

(SDH/PTN) in the access network links provide reliable loop protection, automatic replace and

fiber optic link management function, C-RAN architecture in the access network must also

provide comparative reliability and manageability. In traditional RAN architecture, each BBU on

the access ring usually has access to the corresponding transmission equipment of the center

transmission machine room through SDH/PTN. Through the SDH/PTN ring routing and

app:ds:jitter

app:ds:jitter


protection function, the system can quickly switch to the safe routing mode when any point on

this loop experiences optical fiber failure, ensuring that business is not interrupted. Under the

C-RAN architecture, it also should offer a similar optical fiber ring network protection function.

Centralized BBU should support more than 10~1000 base station sites, and then the optical

fiber connected OBRI link between distributed RRH and centralized BBU is long. If only point-2-

point optical fiber transmission occurred between each distributed RRH and centralized BBU,

then any fault on the optical fiber link will lead to the corresponding RRH loosing service. In

order to ensure the normal operation of the whole system under the condition of any single

point of failure in the optical fiber, the CPRI/Ir/OBRI link connecting the BBU-RRH should use

fiber ring network protection technology, using the main/minor optical fiber of different

channels to realize CPRI/Ir/OBRI link real-time backup.

Operation and Management

At the same time, under the traditional RAN architecture, the transmission network which

consists of SDH/PTN also provides the unified optical fiber network management ability for the

access ring. This includes unified management of the access ring fiber optic link of the entire

network, supervisory control of the access ring optical fiber breakdown, etc. BBU-RRH wireless

signal transport directly on the access ring, whose CPRI/Ir/OBRI interface should also, provides

similar management ability and fit into unified optical fiber network management.

Cost Requirements

Finally, in terms of cost, the high speed optical module necessary for the CPRI/Ir/OBRI optical

interface will be amongst the important factors affecting the C-RAN economic structure.

Compared to traditional architecture, the wireless signal transmission data rate on C-RAN is

more than 100-200 times higher than the bearer service data rate after demodulation. Building

the fiber transportation network in developed city is very hard. This is less of an issue for

operators that already deploy optical fiber and particularly for operators own their own optical

network.

Although the cost of the optical fiber employing CPRI/Ir/OBRI for high speed wireless signal

transmission doesn't need to increase, the high speed optic module or optical transmission

equipment costs must compare to traditional SDH/PTN transmission equipment in order to

make C-RAN architecture more attractive on the CAPEX and OPEX fronts .Therefore, how to

achieve a low cost, high bandwidth and low latency wireless signal optical fiber transmission will

become a key challenge for realization of the future LTE and LTE network deployment by C-RAN.

For the above problems and corresponding technical progress trend, we will analyze and put

forward ideas for solving these problems.

4.1.1 Data Compression Techniques of CPRI/Ir/OBR Link

In view of the above LTE/LTE-A BBU-RRH wireless signal transmission bandwidth problems,

several data compression techniques that can reduce the burden on the OBRI interface are

being investigated to deal with the inevitable bandwidth issue, including time domain

schemes (e.g. reducing signal sampling, non-linear quantization, and IQ data compression)

as well as frequency domain schemes (e.g. sub-carrier compression).

For LTE system with 20MHz bandwidth, the BBU uses 2048 FFT / IFFT but the effective

number of subcarriers is only 1,200, so if the FFT / IFFT is implemented in the RRH, then

the Ir interface between BBU and the RRH only has to transmit effective data subcarriers,

such that the Ir interface load can be reduced about 40%, However, frequency domain

compression leads to an increase in IQ mapping complexity, which would increase the

interface logic design and processing complexity. Meanwhile, the RRH needs to process

app:ds:minor

app:ds:management%20ability


parts of the RACH, Therefore, RRH cannot treat different RACH configurations transparently,

instead RRH needs to process RACH based on configuration. Since there are hundreds of

different configurations, each has to be controlled by different timing algorithms in the RRH,

which could greatly increase the complexity of system design. Therefore, considering the

implementation complexity and cost, such frequency domain compression is not feasible at

the moment.

DAGC time-domain based compression technology is a method used for IQ compression.

The basic principle of DAGC is to select the average power reference based on the best

baseband demodulation range, normalize the power of each symbol, and reduce the signal

dynamic range. DAGC compression will adversely affect system performance. The receiver

dynamic range of the uplink will be reduced, which leads to deterioration of the signal to

noise ratio. At the same time, the EVM indicators will worsen on the downlink. With

increased compression ratio, the system performance will deteriorate even more. Currently,

we still need to investigate the impacts caused by different compression schemes.

Table 2 lists the advantages and disadvantages of various compression schemes. As

indicated, there is no ideal OBRI link data compression scheme. More studies in this area

are required.

Table 2. Comparison of Pros and Cons for Various Data Compression Techniques

Bandwidth

Compression

Schemes

Pros Cons

Reducing signal

sampling

Low complexity;

Efficient compression to 66.7%;

Less impacts on protocols.

Severe performance loss.

Non-linear

quantization

Improve the QSNR;

Mature algorithms available, e.g. A law

and U law;

High compression efficiency to 53%.

Some impacts on the OBRI interface

complexity.

IQ data

Compression

Potential high compression efficiency;

Only need extra decompression and

compression modules.

High complexity;

Difficult to set up a relativity model;

Real-time and compression distortion

issues;

No mature algorithm available.

Sub-carrier

Compression

High compression efficiency to 40%

~58%;

Easy to be performed in downlink.

Increase the system complexity;

Extra processing ability on optical chips

and the thermal design;

High device cost;

Difficulty for maintenance;

RACH processing is a big challenge; More

storage, larger FPGA processing

capacity.

4.1.2 Transmission delay and jitter of CPRI/Ir/OBRI link

As mentioned previously, CPRI/Ir/OBRI link have strict demands on transmission delay,

jitter and measurement. However, because the link round trip delay requirements (5 us) of

the user plane data (IQ data) in CPRI/Ir/OBRI link do not include the transmission medium

round-trip time (i.e. delay in optical transmission), this requirement can be satisfied by the

existing technical conditions. At the same time, because CPRI/Ir/OBRI optical fiber routing

generally does not change with time and delay jitter caused by transmission is relatively

small, it is easy to meet the corresponding requirements.


On the other hand, because LTE/LTE-A has strict requirements about physical layer

treatment delay, CPRI/Ir/OBRI total transmission delay on the link should not exceed a

certain level. The physical layer HARQ process places the highest demand on processing

delay. HARQ is an important technology to improve the performance of the physical layer,

its essence is testing the physical layer on the receiving end of a sub-frame for correct or

incorrect transmission, and rapid feedback ACK/NACK to the launching end physical layer,

then let launching physical layer to make the decision whether or not to send again. If sent

again, the receiver does combined processing for multi-launching signal in the physical

layer, and then provides feedback to the upper protocol after demodulation success.

According to the LTE/LTE-A standard, the ACK/NACK HARQ on uplink and downlink process

should be finished in 3 ms after receiving the signals in the shortest case, which requires

that sub-frame processing delay in the physical layer should be generally less than 1 ms.

Because the physical layer processing itself takes 800-900 us, then CPRI/Ir/OBRI optical

transmission delay may be 100-200 us at the most. According to the light speed(200,000

kilometers per hour) estimated in the fiber, CPRI/Ir/OBRI interface maximum transmission

distance under the C-RAN framework is limited from 20 km to 40 km. Specific value is

related to delay margin the physical layer treatment itself.

4.1.3 Optical Transmission Technology Progress and Cost Reduction

As mentioned above, BBU-RRH wireless signal connection supporting LTE and LTE-Advance

creates new challenges to optical transmission network rates and cost. The rapid

development of the optical transmission technology provides more economic solutions to

solve the problem. A single fiber capacity of current commercial WDM system can be up to

3.2 T.10 Gpbs optical transmission technology applies generally and become fundamental，

40 G system is mature and gradually being commercialized, 100 G technology is still not

mature and costs too much, there is still 2-3 years until the telecommunication

commercial level, but along with coherent technical breakthroughs, promoting of

standardization has already become a now advantage. 10GE standardization and

industrialization will greatly improve the relevant market capacity of the optical

transmission module, which will help to reduce the cost of 10 Gbps optical modules. 40GE

technology is still in the research process. On the other hand, at the access network level,

1.25 G,2.5 G EPON is already widely used in solving FTTX access, 10G PON technology can

be commercial in one or two years, the future PON technological development have several

directions like WDM-PON, Hybrid PON and 40G PON.

Similar to what the Moore's Law is doing in the transformation of the semiconductor

industry, the field of optical communication has a similar trend: Every year, the speed of

optical transmission increases while the cost of the said module declines. Transceiver

modules that are capable of supporting multi-wavelength WDM have emerged in the

market place. Since commercial LTE deployment has just begun, we can safely predict that

it will take about 5 years before the commercial LTE-A multi-carrier system deployment is

needed. By then, if the optical module advancement and cost reduction has reached an

acceptable level, then the RRH-BBU bottleneck will be effectively removed.

Figure 11 shows the 2.5G SFP and 10G SFP / XFP / XENPAK optical modules pricing trends.

We can deduce that optical modules pricing has dropped by 66% to 77% in nearly 3 years,

and the trend will continue in the coming years, further reducing the cost of optical

transmission network. If this price trend continues, it would greatly help to reduce CAPEX

of a C-RAN network.

app:ds:specific


0

500

1000

1500

2000

2500

3000

Aug-07 Feb-08 Aug-08 Feb-09 Aug-09

Pri

ce h

isto

ry o

f 2

.5G

mo

dule

s (

RM

B).

10Km 40Km 80Km

↓66.7%

↓54.2%

↓62.2%

↓60%

↓61.5%

↓35.2%

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Aug-07 Feb-08 Aug-08 Feb-09 Aug-09Pri

ce h

isto

ry o

f 1

0G

mo

dule

s (

RM

B).

550m 10Km 40Km

Fig. 11 Price history of Commercial 2.5G/10G Optical Modules

4.1.4 BBU-RRH Optical Fiber Network Protection

Although BBU-RRH direct transmission under C-RAN framework does not provide a ring

network protection function like traditional SDH/PTN, the CPRI/Ir/OBRI interface rate

standards provide a similar ring network protection function, and are supported by

manufacturers. At the same time, in order to avoid having every RRH fully occupy two

optical fibers on a physically routed pair the RRH‟s can be connected to each in a cascaded

manner according to the CPRI/Ir/OBRI interface specification. This permits two different

routing trunk cables to form a ring and be connected to the same BBU, as shown in Figure

10. As long as the CPRI/Ir/OBRI interface rate is high enough, the BBU-RRH ring network

protection technology can save the use of many optical fibers and ensure a short round trip

delay. Taking a TD-SCDMA system for example, a 6.144 Gpbs CPRI/Ir/OBRI link can

support 15 TD-SCDMA carriers of 8-antenna RRH and a typical TD-SCDMA macro station

with 3 sectors, 5/5/5 configuration at most. The IQ data of a RRH with three sectors

connected to the same BBU machine through two different physical routing backbone

optical cables. When a trunk cable fails, three RRHs will connect to the BBU through

another trunk cable under less than 40ms protection rotated time to guarantee that all

business does not interrupt. For lower-rate GSM system, it is even simpler to connect six or

more RRHs through such a CPRI/Ir/OBRI annular link and achieve the same functions.

However, according to LTE/LTE-A system with higher wireless signal transmission rate, it is

necessary to introduce WDM technology to realize a similar loop protection function.

Transmission ring

Trunk cable 2

Trunk cable 1

Optical

switching box

Central apparatus

room

Radio remote

head

Fig. 12 RRH Ring Protection Loop


4.1.5 Current Deployment Solutions

In order to meet the high bandwidth transmission between RRH and BBU, operators can

use different solutions based on their current transmission network resources. In China

Mobile, the current backhaul is mainly an optical transport network with three layers of

transmission network: the core transmission layer, the convergence transmission layer and

the access transmission layer. All the layers are using ring topology to provide fail safe

protection. The optical resources of different layers are similar to the following: at the core

transmission layer, each optical route has 144 to 576 fibers; at the convergence

transmission layer, each route has 96-144 fibers; while at the access transmission layer,

each route has 24-48 fibers. If the Baseband pool is located in the transmission

convergence equipment room, the optical fiber resource to and from the equipment room

determines the coverage of the baseband pool.

According to the resourcing of the optical transmission network, especially the fiber

resource in the access transmission network, there are four different solutions to carry

CPRI/Ir/OBRI over it: 1. Dark fiber; 2. WDM/OTN; 3. Unified Fixed and Mobile access like

UniPON; 4. Passive WDM. These solutions have different advantages and disadvantages,

and they are each suitable for different deployment scenarios. From the trials conducted,

for a BBU pool with less than 10 macro BSs, it is preferred to use a dark fiber solution while

other solutions still need more field tests and verification, because they may introduce new

transmission devices and associated O&M issues.

The first solution is Dark fiber. It is suitable when there is plenty of fiber resource. It is easy

to deploy if there are a lot spare fiber resources. The benefits of this solution are: fast

deployment and low cost because no additional optical transport network equipment is

needed. The concerns of this solution are: it consumes significant fiber resource, thus the

network extensibility will be a challenge; new protection mechanisms are required in case

of fiber failure; and it is hard to implement O&M, therefore it will introduce some difficulties

for optical network O&M. However, there are feasible solutions to address such challenges.

For fiber resources, if there is already a channel route available, it is fairly inexpensive to

add new fiber cables or upgrade existing fibers. To address fiber failure protection, there

are CPRI/Ir/OBRI compliant products available now that have the 1+1 backup or ring

topology protection features. If deployed with physical ring topology that provides

alternative fiber route, it will be able to provide similar recoverability capability as SDH/PTN.

For the O&M of the fiber in the access ring, we are considering introducing new O&M

capabilities in the CPRI/Ir/OBRI standard to satisfy the fiber transport network

management requirement.

The second solution is WDM/OTN solution. It is suitable for Macro cellular base station

systems when there is limited fiber resource, especially where the fiber resource in the

access ring is very limited, or adding new fiber in existing route is too difficult or cost is too

high. By upgrading the optical access transmission network to WDM/OTN, the bandwidth of

transporting CPRI/Ir/OBRI interface on BBU-RRH link is largely improved. Through

transmitting as many as 40 or even 80 wavelength with 10Gpbs in one fiber, it can support

a large number of cascading RRH on one pair of optical fiber. This technology can reduce

the demand of dark fiber, however, upgrading existing access ring into WDM/OTN

transmission network means higher costs. On the other hand, because the access transport

network is usually within a few tens of kilometers, the WDM/OTN equipment can be much

cheaper than those used in long distant backbone networks.

The third solution is based on CWDM technology. It combines the fixed broadband and

mobile access network transmission at the same time for indoor coverage with passive


optical technology, thus named as „Unified PON‟. It can provide both PON services and

CPRI/Ir/OBRI transmission on the same fiber [5]. In this solution, an optical fiber can

support as many as 14 different wavelengths. In the UniPON standard, the uplink and

downlink channel are transmitted on two difference wavelengths, thus other free

wavelengths can be used for CPRI/Ir/OBRI data transmission between the BBU and RRH.

Because of sharing the optical fiber resources, it can reduce the overall cost. It is suitable

for C-RAN centralized baseband pool deployment of indoor coverage.

4.1.6 Summarize

Based on the above analysis, „fully centralized‟ C-RAN architecture requires a high

bandwidth, low latency, high reliability and low cost optical solution to transmit high speed

baseband signal between BBU and RRH. It‟s promising to find feasible solutions emerging in

the near future. However, there are still many challenges in the current solutions. For

example, current data compression schemes fail to satisfy OBRI transmission in the LTE-A

phase. The rapid development of high-speed optical modules and the associated cost

reduction is heading in the right direction but we still need a breakthrough in optical devices.

Failure protection schemes for BBU-RRH connection are able to provide similar functions to

SDH/PTN in case of fiber cut, but we still need to find solutions for unified O&M with

traditional transmission networks. UniPON based on passive WDM technology is a promising

solution for certain deployment scenarios but it must be designed to be competitive in cost.

In conclusion, we have various directions to solve the high-speed baseband signal

transmission requirement of C-RAN but we still need to explore new technology or a

combination of existing technology to find a more economical and effective solution.

Considering the technical challenges as well as the limitation in current optical network

resources, it is clear that C-RAN can be widely applied in a short time frame. Instead, a

stepped plan should be used to gradually construct the centralized network: first,

centralized deployment can be applied in some green field or replacement of old network in

a small scale. Dark fiber can be used as the BBU-RRH transmission solution. One access

ring that connects 8~12 macro sites can be centralized together, with a maximum ring

range of 40km. In the future, a larger number of macro BS in various deployment scenarios

can be further tested.


4.2 Dynamic Radio Resource Allocation and Cooperative

Transmission/Reception

One key target for C-RAN system is to significantly increase average spectrum efficiency and

the cell edge user throughput efficiency. However, users at the cell boundary are known to

experience large inter-cell interference (ICI) in a fully-loaded OFDM cellular environment, which

will cause severe degradation of system performance and cannot be mitigated by increasing the

transmit power of desired signals. At the same time, in view of the analysis, single cell wireless

resources usage efficiency is low. To improve system spectrum efficiency, advanced multi-cell

joint RRM and cooperative multi-point transmission schemes should be adopted in the C-RAN

system.

Cooperative Radio Resource Management for multi-cells

The multi-cell RRM problem has been addressed in various academic studies. Many uses

various optimization techniques in trying to determine the optimal resource scheduling and the

power control solutions to maximize the total throughput of all cells with some specific

constraints. To reduce the complexity incurred in the C-RAN network architecture and the

scheduling process, the joint processing/scheduling should be limited to a number of cells

within a “cluster”. The complexity of scheduling among the eNBs clusters is determined by the

velocity of mobile users and the number of UEs and RRHs in the cluster. Thus, choosing an

optimal clustering approach will require balancing among the performance gain, the

requirement of backhaul capacity and the complexity of scheduling.

As shown in Fig.13, UEs will be served by one of the available clusters which are formed in a

static or semi-static way based on the feedback or measurements reports of UEs. In this

scenario, a subset of cells within a cluster will cooperate in transmission to the UEs associated

with the cluster. To further reduce the complexity, it is possible to limit the number of cells

cooperating in joint transmission to a UE at each scheduling instant. The cells in actual

transmission to a UE are called active cells for the UE. The active cells can be defined from the

UE perspective based on the signal strength (normally cells with strong signal strength are

chosen among cells within the supercell). The activation/de-activation of a cell can be done by

a super eNB, which is the control entity in cell clustering and can adjust the sets scope based

on the UE feedback.

Cell cluster 1Cell cluster 2

Cell cluster 3

Fig. 13 The UE assisted network controlled cell clustering


Cooperative Transmission / Reception

Cooperative transmission / reception (CT/CR) is well accepted as a promising technique to

increase cell average spectrum efficiency and cell-edge user throughput. Although CT/CR

naturally increases system complexity, it has potentially significant performance benefits,

making it worth a more detailed consideration. To be specific, the cooperative transmission /

reception is characterized into two classes, as shown in Fig.14:

Joint processing/transmission (JP)

The JP scheme incurs a large system overhead: UE data distribution and joint

processing across multiple transmission points (TPs); and channel state information

(CSI) is required for all the TP-UE pairs.

Coordinated scheduling and/or Coordinated Beam-Forming (CBF)

With a “minimum” cooperation overhead, to improve the cell edge-user throughput via

coordinated beam-forming: No need for UE data sharing across multiple TPs; Each TP

only needs CSI between itself and the involved UEs (no need for CSI between other

TPs and UEs).

Fig. 14 JP scheme and CBF scheme

In this section, the performance of the JP scheme with intra-cell collaboration, and performance

with inter-cell collaboration in C-RAN architecture are evaluated in a TDD system. We assume

that full DL channel state information (CSI) can be obtained ideally at the eNB side. The

downlink throughput and spectrum efficiency results with different schemes in both 2 antenna

and 8 antenna configuration are shown in Fig.15. Detailed simulation parameters can be found

in [6-9].

3GPP Case 1 (TDD)

1.9 2.542.47

5.46

2.81

6.15

3.01

6.58

0

2

4

6

8

2Tx (X)/2Rx 8Tx(XXXX)/2Rx

Ave. cell spectrum efficiency (bps/Hz)

SU-MIMO

MU-MIMO

0.0560.098

0.047

0.1830.0780.227

0.101

0.266

0

0.1

0.2

0.3


Cell-edge spectrum efficiency (bps/Hz)

SU-MIMO

MU-MIMO


ITU UMi (TDD)

1.44 1.971.8

3.78

1.93

4.54

1.97

5.35

0

2

4

6


Ave. cell spectrum efficiency (bps/Hz)

SU-MIMO MU-MIMO

Intra-site CoMP C-RAN CoMP

0.0410.0520.039 0.092

0.07 0.1610.075

0.202

0

0.1

0.2

0.3


Cell-edge spectrum efficiency (bps/Hz)

SU-MIMO

MU-MIMO

Fig. 15 Compare of Downlink Throughput and SE

From the simulation results we can see, compared to the non-cooperative transmission

mechanism (MU-BF in LTE-A), the spectrum efficiency of intra-cell collaboration and inter-cell

collaboration under C-RAN architecture could achieve a 13% and 20% gain, respectively, while

the cell edge user‟s spectrum efficiency, from the above two mechanisms can get 75% and 119%

gain respectively.

Technical Challenges

Cooperative transmission / reception (CT/CR) has great potentials in reducing interference and

improving spectrum efficiency of system. However, this technology has many problems that

need to be further studied before it can be applied to the practical networks. There are many

challenges listed as follows:

Advanced joint processing schemes

DL channel state information (CSI) feedback mechanism

User pairing and joint scheduling algorithms for multi-cells

Coordinated Radio resource allocation and power allocation schemes for multi-cells.

4.3 Large Scale Baseband Pool and Its Interconnection

Centralized Baseband Pool

There are many distributed BS products using RRH+BBU architecture in market. Some TEM‟s

products have realized dynamic allocation of carrier processing within one BBU to adapt to

dynamic workloads among different RRH connected to it. This architecture can be viewed as the

first step of centralized baseband pool concept, but in general a single BBU has limited

processing capability, typically only supporting about 10 macro BSs‟ carriers. It‟s not yet

capable of supporting dynamic resource allocation across different BBU, thus hard to resolve

the dynamic network load in a larger area. In the current RRH+BBU architecture, the RRH is

usually connected to a particular BBU by a fixed link, and it can only transmits its baseband

signal and O&M signaling to the BBU it‟s connected to. This makes it difficult for another BBU to

obtain any uplink baseband data from that RRH. Similarly, any other BBU has difficulty sending

downlink baseband data to this RRH. Because of this limitation, the processing resources of

different BBUs can hardly be shared: the idle BBU‟s processing resources are wasted and it

cannot be used to help the BBU with a heavy workload.

The centralized baseband pool should provide a high bandwidth, low latency switch matrix with

an appropriate protocol to support the high speed, low latency and low cost interconnection

among multiple BBUs. In a medium sized dense urban network coverage (approximately 25 sq.


km in area), with an average distance between BS of 500m, a centralized baseband pool that

can cover the whole area needs to support about 100 BS. For a typical TD-SCDMA system with

3 sectors per macro BS and 3 carriers/sectors, it means that the centralized baseband pool

needs to support 900 TD-SCDMA carriers. Imagine if the centralized Baseband pool coverage

is even larger, such as 15 km X 15 km, then the baseband pool would need to support up to

1000 macro BSs‟ carriers. Because of the limitation in the high-speed differential signal

transmission, the traditional BBU architecture cannot scale up to support such capacity by

simply expanding the backplane dimensions.

Infinite Band technology can provide significant switching bandwidth (20Gbps-40Gpbs/port)

and very low switching latency. It is widely used in supercomputers. However, the cost per port

is very high (20,000RMB) and as such does not meet the C-RAN cost requirement. Inspired by

the data center network‟s distributed inter-connect architecture, the centralized BBU pool in C-

RAN can also use a distributed optic interconnection to combine multiple BBU into a scalable

baseband pool. Based on that, the RRHs‟ signal can be routed to any one of BBUs in the pool.

Thus load balance according to dynamic network load among BBUs can be achieved, and

system power consumption can be reduced. It also makes the deployment of multi-point MIMO

technology and interference mitigation algorithms easier, which can improve radio system

capacity.

Dynamic carrier scheduling

The dynamic carrier scheduling of resources within baseband pools enhances redundancy of the

BBU and increases overall operational reliability of the baseband pool. When a baseband card or

a carrier processing unit fails, the work load can be promptly redistributed to other available

resources within the pool, and restore the normal operation. In addition, for areas that have

strong dynamic network load, the operator can deploy fewer baseband resources to meet the

demands of different sites that have opposite peak loads at different times. For example,

operator can use the same BBU pool with multiple RRHs to cover both residential areas and

office areas. Then dynamically allocates baseband resources to ensure basic coverage for both

areas. Remaining baseband resources can be dynamically allocated to cover the business area

during working hours and the residential area during after working hours. This will increase the

overall carrier resource utilization.

Large-scale BBU Inter-connection

A large scale baseband inter-connect solution should be able to support 10-1000 macro BS,

with the following requirements:

Inter-connection between BBUs must satisfy the wireless signal‟s requirements of low

latency, high speed, and high reliability. The requirements are similar to the CPRI/Ir/OBRI

interface, and should support real-time transmission of 2.5/6.144/10Gbps rate.

Dynamic carrier scheduling among BBUs to achieve efficient load balance within the

system and failure protection without service interruption.

Support multipoint collaboration (CoMP). It needs to consider the data flow between

different BBUs to support collaboration radio.

Fault-tolerance. Fiber inter connection should support 1+1 failure protection, BBU frame

and baseband processing board N +1 protection to achieve high system robustness.

High scalability: it can extend the system capability smoothly without services interruption.


4.4 Open Platform Based Base Station Virtualization

Current Multi-Standard BS Solutions

Nowadays, most major mobile operators in the world have to operate multiple standards

simultaneously. It is a natural choice to use multi-mode base stations for low cost operation.

Therefore, SDR based on a common platform to support multi-standards has become the

mainstream in TEM‟s products. The following are the two types of multi-mode base stations.

Unified BBU system platform supporting multi-mode by plugging in different processing

boards. The processing board which supports multi-standard (such as GSM, TD-SCDMA,

TD-LTE) has a unified interface and can be plugged in the same BBU system platform.

Operators can use one set of a BBU system platform to support multi-standard operation.

In this case, some modules of BBU system such as control module, timing module and RRH

I/O modules can be shared between BBU processing boards which support different

standards. However, this structure can't share processing resources between different

processing boards and usually need to replace or add new processing board hardware for

upgrades.

Unified BBU system platform and unified processing board hardware platform to support

multi-mode through the software re-configuration. Through software upgrades or

configuration, the same processing board can support different standards (e.g. LTE or TD-

SCDMA). In some of the latest products, the RRH can also be SDR-enabled to support

different standards in the same spectrum band. This solution allows the base station to be

upgraded to a new standard without changing the hardware. However, current products

usually require the BBU to restart in order to download new DSP / FPGA software for

standards upgrade. This limits the sharing of hardware between different standards. In

fact, this prevents the dynamic resources allocation according to real-time traffic load

without interrupt of services.

Current SDR base station products partially meets the requirements of multi–standards support,

however, it does not satisfy the operator flexible operation requirement of dynamically shared

resources among multiple standards, load-balancing, etc.

Evolution of Software Defined Radio

Driven by Moore's law in semiconductor industry, Digital Signal Processor (DSP) and General

Purpose Processors (GPP) have made a lot of progresses in the architecture, performance and

power consumption in recent years. This provides more choices for SDR base stations. Multi-

core technology is widely used in DSP and 3 ~ 6 cores processors have been commercially

available. At the same time, DSP floating-point processing capacity is also improving at a fast

pace. The emergence of the DSP system based on SoC architecture combines traditional DSP

core and communication accelerator together has improved the BBU processing density and

improved the power efficiency. Moreover, real-time OS running on DSP pave the path to

virtualization of DSP processing resources. On the other hand, DSP from different

manufacturers and even a same manufacturer cannot guarantee backwards compatibility. The

real-time operating systems are different from each other, and there is no de fact standard yet.

Generally BBUs based on DSP platform are proprietary platforms. And it is still difficult to

achieve smooth upgrading and resource virtualization.

Meanwhile, General Purpose Processors have progressed rapidly, and they are now capable of

efficiently processing wireless signals. Therefore, the telecom industry now has more choices

for software defined radio. Technology evolution in areas such as multi-core, SIMD (single-

instruction multiple data), large on-chip caches, low latency off-chip system memory are

facilitating the use of GPP in traditional signal processing applications such as baseband


processing in base stations. Traditional general processors usually have lower performance than

DSP in power efficiency; however, in recent years the general processor has made a lot of

improvements in this respect. Fig.14 shows the general processor technical progress in

processing performance and power consumption in nearly 6-7 years. It is can be seen that the

floating point computing capacity per watt improves very fast. These data points prove that the

evolution in GPP has made it an attractive solution for various data processing tasks in the base

station.

The advantage of GPP is that they have a long history of backward compatibility, ensuring that

software can run on each new generation of processor without any change, and this is

beneficial for smooth upgrade of the BBU. On the operating system side, there are multiple

OS‟s available on GPP that have real-time capability, and also allow the virtualization of BS

baseband signal processing.

Fig. 14: Compute performance evolution of GPP *

(CPUs in 50-65 watt power envelopes used as basis for comparison in graph)

Technical progress in DSP and GPP has provided more powerful signal processing with less

power consumption. This progress has made the SDR based BS solutions more attractive.

Traditional DSP has become matured solution for product, and will continue to evolve. The

advanced research on wireless signal processing on GPP has provided more choices for the base

station, and has the potential to become part of the future open, unified multi-mode BS

platform.

Base Station Virtualization

Once the large scale BBU pool with high-speed, low-latency interconnection, plus the common

platform of DSP/GPP and open SDR solution could be realized, it has set the base for a a virtual

BS.

Virtualization is a term that refers to the abstraction of computer resources. It hides the

physical characteristics of a computing platform from users, instead showing another abstract

computing platform. If such a concept can be utilized in a base station system, the operator

can dynamically allocate processing resources within a centralized baseband pool to different

virtualized base stations and different air interface standards. This allows the operator to

efficiently support the variety of air interfaces, and adjust to the tide effects in different areas

and fluctuating demands. At the same time, the common hardware platform will provide cost

effectiveness to manage, maintain, expand and upgrade the base station. Therefore, we believe


real time virtualized baseband pools will be part of the next generation wireless network, as

shown in Fig. 15. Within in given centralized baseband pool, all the physical layer processing

resources would be managed and allocated by a real time virtualized operating system. So, a

base station instance can be easily built up through the flexible resource combination. The real

time virtualized OS would adjust, allocate and re-allocate resources based on each virtualized

base station requirements, in order to meet its demands.

Processors

Processors

Processors

Processors

…

Physical Hardware

PHY Layer

(Signal processing)

resource pool

MAC/Trans. Layer

(Packet processing)

resource pool

Accelerator

(CODEC, cryto, etc.)

resource pool

Control & Manage

(O&M processing)

resource pool

CC AA MM PP

Base station Virtualization Base station Instances

BS of standard 1

CC AA MM PP

BS of standard 2

CC AA MM PP

BS of standard 3

Fig. 17 Baseband Pool

All the adjustments will be done by software only. With this mechanism, the base stations of

different standards can be easily built up through resource reconfigure in software. Also,

cooperative MIMO can get the required processing resources dynamically. In addition, the

processing resources can be assigned in a global view, thus the resource utilization can be

improved significantly.

Technical Challenges

Since wireless base stations have stringent real-time and high performance requirements,

traditional virtualization technique is challenged to solve the latency requirements of wireless

signal processing. In order to implement real time virtualized base station in a centralized base

band pool, the following challenges have to be solved:

High-performance low-power signal processing for wireless signals.

General purpose processor and advanced processing algorithm for real time signal

processing

The high-bandwidth, low latency, low cost BBU inter-connection topology among physical

processing resources in the baseband pool. It includes the interconnection among the chips

in a BBU, among the BBUs in a physical rack, and among multiple racks.

Efficient and flexible real-time virtualized operating system, to achieve virtualization of

hardware processing resources management, and dynamic allocation of physical

processing resources to each virtual base station, in order to ensure processing latency

and jitter control HW level support on virtualization in order to minimize latency.


4.5 Distributed Service Network

DSN builds the elastic high-capacity switch system adopting P2P technology, which ensures

high system reliability based on disaster tolerance and auto recovery technology in software

implementation. By using self-organization and self-adapting technology, in conditions of

capacity expansion, equipment failure or overload, the configuration can be completed

automatically with little manual work, thus reducing OPEX.

DSN can replace traditional carrier-class equipment with a general purpose server, and DSN

introduces virtualization technology, the DSN nodes are encapsulated in VM(Virtual Machine),

through VM live migration, when the traffic goes down, multi DSN nodes can aggregate to a

few physical servers, and other servers can be turned off, thus implementing energy

conservation and emission reduction.

Distributed Service

Network

BBU pool

BBU pool

DSN element

C-RAN element

Fig. 18 C-RAN Integrated with DSN

In a platform layer, DSN and C-RAN both encapsulate their network elements through

virtualization technology on general servers, so, it is possible to run DSN and C-RAN on the

same virtualized platform. But how to implement the resource management（ including the

dimension of time and the dimension of physical resource ）is the key issue in the research of

platform unification for DSN and C-RAN.


5 Evolution Path

The novel C-RAN architecture is a revolution of the traditional RAN deployment. It is impossible

to replace today‟s RAN overnight. Moreover, the technical challenges of C-RAN should be

carefully developed and tested in labs and field environments to ensure its reliability. This

naturally leads to a step-by-step evolution path of C-RAN to gradually replace traditional RAN.

The following is our vision on how the evolution could take place:

5.1 C-RAN Centralized Base Station Deployment

In the first step, Base Stations can be implemented by separating Remote Radio Heads (RRH)

and Baseband Units (BBU), and baseband processing resources between multiple BBUs in a

centralized Base Station can be scheduled in carrier level. The RRHs are small and light weight

for easier deployment. They receive/transmit digital radio signals from/to the BBU via fiber

links such as CPRI/Ir/OBRI. The BBU is the core of the radio signal processing. RRHs can be

deployed in remote sites far from the physical location of the BBU (e.g. 1~10km). The optical

fiber transmission network between RRH and BBU shall have the corresponding loop protection

and management functions. The fiber link between RRHs and BBUs can be standardized like

CPRI/Ir/OBRI so that RRHs and BBUs from multiple venders can be connected together.

The centralized BS has a high bandwidth, low latency switch matrix and corresponding protocol

to support the inter-connection of carrier processing units among multiple BBUs in order to

constitute a large-scale baseband pool. The signals from distributed RRH can be switched to

any BBU inside the centralized baseband pool. Thus, the centralized baseband pool can realize

carrier load balance to avoid some BBUs overloaded while some BBUs idle, and realize fault-

tolerance to avoid that the fault of single BBU affect the overall functions and coverage of the

wireless network. The above technologies can improve the usage efficiency of devices, reduce

power consumption and improve system reliability.

5.2 Multi-standard SDR and Joint Signal Processing

In the second step, on the basis of centralized BS deployment, the BBU‟ baseband processing

functions can be fully implemented by Soft Defined Radio (SDR) based on a unified, open

platform. By moving the baseband processing to SDR, it is much easier to support multiple

standards, upgrade the SW/HW, introduce new standard and increase system capacity.

Meanwhile, with multiple RRHs attached to the centralized BBU pool, it is easier to implement

coordinated beamforming (CBF) and cooperative multipoint processing (CoMP) in this platform.

Multiple BBUs can coordinate with each other to share the scheduling information, channel

status and user data efficiently to improve the system capacity as well as reduce interferences

in system.

5.3 Virtual BS on Real-time Cloud Infrastructure

Once centralized baseband pool consisted of a lot of standardized BBUs is built on a unified,

open platform and the baseband processing is implemented by SDR, the virtual BS based on

real-time cloud infrastructure is the next step of C-RAN evolution.

The centralized baseband pool consisted of large-scale BBUs by a high bandwidth, low latency

network, combined with some system software, can constitute a large „real time baseband

cloud‟, just like the cloud computing environment in IT industry. The difference is that the

baseband processing tasks are real-time computing tasks in a real time baseband pool.


Through the cooperation of BBU in the baseband pool and RRH to send and receive wireless

signals, it can be achieved that multi-standard wireless network functions in the same platform.

In the system software instructions of the baseband pool based on real-time cloud architecture,

CPRI/Ir/OBRI optical fiber transmission network and optical Internet architecture in large-scale

centralized baseband pool can send the baseband signal signals transmitted by RRH to the

virtual base station running on the designated BBU. Then virtual base station uses the

calculation resources of the designated BBU to finish the real-time processing of wireless

baseband signals. Moreover, in a C-RAN system which has several baseband pools,

CPRI/Ir/OBRI optical fiber transmission network should have the ability to forward the

baseband signals from RRHs to other baseband pools in order to improve system reliability and

realize load balance across different baseband pools.


6 Recent Progress

To accelerate the development and commercialization of C-RAN, China Mobile has been working

actively with industry partners. We have made good progress in field trial, large scale BBU pool

implementation, and baseband PoC based on IT platform. This chapter will first introduce the

advantages and disadvantages observed in C-RAN field trial, followed by discussion of large

scale BBU pool solution, up to 1000 carriers, based on current BBU device, and lastly the recent

R&D result of multi-mode PoC based on IT platform.

6.1 TD-SCDMA and GSM Field Trial

China Mobile conducted the first C-RAN trial with partners in 2010. It is a C-RAN centralized

deployment field trial within the commercial TD-SCDMA system in Zhuhai city, Guangdong

province. After that, there has been multiple GSM field trials conducted in multiple cities

throughout China, include Changsha, Baoding, Jilin, Dongguan, Zhaotong, etc. Rest of the

section discusses the pros and cons of the C-RAN centralized deployment solution‟s pros and

cons in different scenarios. For the ease of discussion, two typical cases, TD-SCDMA trial in

Zhuhai city and GSM trial in Changsha city are shown here.

Overall situation

The first trial in Zhuhai City only took 3 months to complete. The commercial trial has 18 TD-

SCDMA macro sites covering about 30 square km area. This trial has verified some centralized

deployment technologies feasibility. The construction and operation of a commercial clearly

highlighted the C-RAN‟s advantage over tradition RAN in cost, flexibility and energy savings. At

the same time, it also exposed challenges on fiber resource, as well as transmission

construction.

After that, there have been several trials on centralized deployment solutions of GSM system.

The network layout is mainly consisted of replacing and upgrading existing sites. There are

total15 sites covering 15 square km in the trial, where only 2 of them are new sites. Compared

with TD-SCDMA network, GSM solutions have unique features, for example, it could support

daisy-chain of 18 RRHs with only 1 pair of fiber. This could significantly reduce the number of

fiber resources needed in C-RAN centralized deployment with dark fiber solution.

The following sections will describe the network status before and after C-RAN deployment, key

technology introduced, field test results and challenges observed. .

Field Trial Area

The trial area in Zhuhai city is mainly consisted of a national high tech development zone, a

residential community, and a few college campuses. The data traffic in this area is growing

rapidly, as the customers here are well-educated and early adapters of new services. Part of

the trial areas has demonstrated tidal effect of traffic loading, with predictable traffic loading

pattern associated with time, location or event. For example, the national high tech

development zone has most people during working hours. The same group of customers usually

returns to nearby community after work. Students in colleges tend to stay away from using

wireless devices during school hours, while they tend to make a lot of calls in night.

Traditionally, network planning must support the peak traffic load at each individual site, which

is usually 10 times higher than the down time This results in a very low average utilization rate

of the BTS devices. It also introduces difficulties in network planning, construction and

optimization. It is suitable to adopt baseband pool with dynamic carrier allocation. In the trial

field, there will be 9 sites co-located with existing GSM site, while another 9 sites is new. All


these 9 sites have to be connected with new fiber channels and they are spread in 30 square

km. This is a challenge for fiber construction.

The trial area in Changsha city is consisted of a few campuses near Yuelu Mountains. The traffic

load and traffic density is quite high here. In addition, there is a lot of dormitories, and local

residential apartments. The propagation environment is very complex and the coverage KPI still

has room to be improved. This makes it suitable to verify C-RAN‟s capacity in urban city

environment. Finally, since most of the trial sites are reusing or upgrading existing ones, there

is plenty of fiber resources.

Overall Solution

The solution starts with planning of system capacity in centralized deployment. In the Zhuhai

trial, each TD-SCDMA site‟s configuration is 4/4/4, which means that there are 3 sectors in

each site, and every sector has 4 carriers. Overall, the 18 trial sites need 216 carriers. When

considering the BBU pool capacity, the total BBU pool can be planned to support the maximum

co-current traffic for the same area.

There are two kinds of TD-SCDMA carriers, R4 carrier is mainly used for voice traffic, and

HSDPA carrier is mainly used for data traffic. Based on China Mobile‟s planning requirements,

every site‟s traffic load should not exceed 75%. As a result, each R4 carrier supports up to 203

voice users, and each HSDPA carrier can support up to 93 users. There are total 17,000

effective users in the trial area. When BBU pool is deployed, 160 carriers will be able to support

20,000 effective users. This means the C-RAN centralized deployment can save the BBU

capacity by roughly 25%, compared with traditional deployment method.

Similarly, the trial in Changsha also has used the co-current capacity to decide the total

capacity of the BBU pool.

The second part of the solution involves dynamic carrier allocation. In TD-SCDMA system, each

RRH/sector can support maximum 6 R4 and HSDPA carriers. In the idle situation, each

RRH/sector has only one R4 carrier and one HSDPA carrier. There are different carrier allocation

decision criteria whether more R4 and HSDPA carriers should be added. Whenever the existing

R4 carrier‟s loading rate is above a threshold, there should be more R4 carriers allocated in this

site. For HSDPA carrier, similar rule applies. Where there is not enough load in multiple R4 or

HSDPA carrier, it is also possible to reduce the number of R4 and HSDPA carriers in one sector.

For GSM system similar rule also applies but the criteria is the utilization rate of each GSM

carrier.

The third portion of the solution involves RRH daisy chain and fiber failure protection

technologies. These technologies are derived from the distributed BBU-RRH deployment

method which usually uses point-to-point dark fiber connections. When BBU-RRHs are

separated by significant distance, it is important to consider the saving of fiber resource and

protection against unpredictable fiber failure caused by external factors. In TD-SCDMA, each

fiber link can handle up to 6.144Gpbs transmission, enough to support 15 TD-SCDMA carriers.

Thus, one pair of fiber is able to support one site with 3 sectors and maximum carrier of 15. In

the Zhuhai trial, each access ring has 9 sites and used 9 pair of fibers to support the 9 sites

connected to the ring.

On the other hand, GSM has far less baseband requirement due to its narrow band nature;

therefore it can support more capacity in daisy-chain configuration. There are commercial

products that can support 18 to 21 RRH daisy chained on one pair of dark fiber. We can

calculate the fiber resource required per access ring as following: usually, each access ring has

8~ 12 physical sites and each site has 3 sectors, and has 900M and 1800M dual bands. This


means, each access ring may has up to 16~24 logical sites, which is 48 to 72 sectors/RRH. To

connect all the RRH in daisy chain, we would need 4~5 pair of fibers in the ring.

Lastly, the field trial has also verified key technology for outdoor deployment, like power supply

for remote sites. In the Zhuhai Trial, there is no BTS equipment room in the 9 new sites. Thus

the traditional DC power supply is not available. External power booth is used instead. Existing

outdoor power solution met the need of network deployment: with sufficient operation

temperature range, -40℃～+70℃, C-level anti-flash capacity and theft-proof solution to ensure

the safety of device without on-site attendance. GSM and TD-SCDMA remote site both can

apply this outdoor power solution.

Technical Performance

This section will outline the technical performance data from selected test cases in the trial,

starting with the dynamic carrier allocation procedure. The following figure illustrates the total

number of carriers allocated to one sector in a typical day on one site in Zhuhai trial. The blue

curve represents this sector‟s total carrier capacity, while the purple curve represents the actual

network load for this sector. It is clearly shown that the dynamic carrier allocation has adapted

effectively to dynamic load in network.

Fig. 19 Dynamic Carrier Allocation

We also collected KPI of radio performance for both dynamic carrier allocation and static carrier

allocation. We noted no KPI difference.

In the Changsha trial, the C-RAN centralized deployment has shown better radio performance

and improved user experience, due to the introduction of co-located multi-RRH per site

technology. With this technology, multiple RRH transmit and receive signals for the same cell,

just like fiber repeat does but provide additional receive combination gain. Multiple radio

performance is improved, include uplink receive quality improved by 2%~3%, drop call rate

was reduced and nearly eliminated in some sites. In addition, since inter-site handover has

become an internal procedure in one BBU pool, the handover delay has been reduced. Finally,

the fiber protection was in place when the access fiber ring was cut accidently, the BBU-RRH

traffic will be automatically switched to another unaffected route in the ring. The switching

delay during the failure protection is comparable to normal cross-BTS or cross-MSC. Thus the

failure protection has very limited effects to network KPI.

In summary, C-RAN centralized deployment does not have negative effect on radio

performance. On the contrary, it may provide extra gains on radio performance. Moreover, RRH

daisy chain could reduce the dark fiber resource needs, while out-door units meet the power

requirement of out-door remote sites. Now dark fiber transportation solution has been well

verified, and other transmission technologies are in testing.


Economic analysis

The trial in Zhuhai city shows that, compared with traditional RAN deployment method, C-RAN

centralized deployment can reduce the TD-SCDMA network‟s CAPEX and OPEX significantly,

especially for new TD-SCDMA site which is not reusing existing GSM site. In the following figure,

it is shown that OPEX and CAPEX can be reduced by 53%, and 30% respectively for new cell

sites.

Fig. 20 Economic analysis for centralized deployment

On OPEX, the savings are mainly come from A/C power consumption, site rental fee, regular

on-site maintenance visit , and reduced human resource on repair and upgrade. The key factor

is C-RAN has only RRH in remote site and no BTS equipment room, the site rental fee is much

lower, and O&M cost is also lower. This is an important saving, as the site rental fee is a

significant portion of the Zhuhai system TCO.

On CAPEX consideration, the savings are mainly from: no new BTS room, reduced transmission

devices on each remote site, and eliminating of various supporting devices in remote site. In

addition, the adaption of BBU pool can reduce the BTS configuration and potentially lower the

CAPEX on RAN.

In GSM trial, similar CAPEX/OPEX savings have been observed. However, it is very clear that

the savings achieved in these two cases are different, due to the different fiber resources,

different deployment scenarios in different city.

All-in-all, the economic analysis has shown the benefits in different areas. It is able to reduce

RAN‟s O&M. however, it may be important to take account of each individual case to better

calculate the saving of CAPEX and OPEX. In addition, RRH requires much less and power, it is

easier to find new site, and easier to move to different place, which largely reduces the risk of

cell sites being forced to relocate due to regulations or neighborhood complaints, and the cost

and service disruptions associated with these.

Construction Impact

The centralized deployment of C-RAN greatly simplifies the remote site selection and

construction requirements, construction time required for new base stations, which lead to

faster network deployment. Table 3 shows the comparison of the construction process between

traditional base station and C-RAN centralized approach in the China Mobile‟s TD-SCDMA

network deployment in Zhuhai City, Guangdong Province.

From figure 17, C-RAN showcases the advantage of deployment time. The savings are mainly

from site selection/purchasing, base station equipment room construction and transmission

system debug, etc.


Table 3. Impact of C-RAN Centralization concept to construction cycle

Process Traditional Base Station

Construction Centralized Base Pool

Construction

site selection Stringent, flexible

Equipment room Site rental and construction No site construction for RRH

Power supply equal

Site Equipment Installation needed No requirement

transmission Installation and verification needed Only verification needed

Equipment install Radio system and BBU RRH and centralized BBU

Verification Distributed BBUs require higher

verification centralized BBUs require less

verification effort

Fig. 21 Construction cycle comparison

Power Consumption Analysis

C-RAN RRH does not require on-site equipment room and associated air conditioner which

reduce electricity cost. Comparing to traditional base station, single RRH can save up to 75.3%

power consumption in the China Mobile‟s TD-SCDMA network deployment in Zhuhai City,

Guangdong Province. The itemized energy saving is listed in table 4.

Table 4. Power consumption comparison

RAN

architecture

Base

Station

equipment

Air

conditioning

Switching

Supply

Storage

Battery

Transmission

System Total

Traditional 0.65 KW 2.0 KW 0.2 KW 0.2 KW 0.2 KW 3.45KW

C-RAN 0.55KW 0 0.2KW 0.10KW 0 0.85KW

Summary

C-RAN centralized commercial access network demonstrates several benefits including: 1)

simplified site selection and improve the speed of location selection negotiations; 2) reduced

base station construction and maintenance cost, improved network deployment efficiency; 3)

reduced supporting facilities of remote cell sites, led to construction cost reduction by 1/3 per

site.

In terms of network operation, C-RAN takes advantage of low cost, energy efficiency RRH.

Centralized BBU facilitates easy maintenance and flexible upgrade. The overall network

utilization can be improved due to virtualization technology and resource sharing which not only

increases utilization but also lowers overall power consumption thru various power

management schemes.


6.2 Large Scale Baseband Pool Equipment Development

In the first half of 2011, China Mobile Research Institute (CMRI) and its C-RAN partners

developed large scale Baseband Pool supporting more than thousands of carriers. The

innovation includes the IQ data routing switch method designed by CMRI, using existing

equipment. Several C-RAN partners have made breakthrough progress to expand the scale of

Baseband Pool beyond thousands of carriers. The large scale of Baseband Pool is based on

distributed multilayer switch architecture, with high serviceability, low maintenance and flexible

capacity expansion. This section describes the key technology for large scale baseband pool

development -- IQ data routing switch, and its adaptive improvement for telecommunication

equipment. Finally, it briefly highlights the key technical characteristics of the equipment.

IQ Data Routing Switch Architecture

IQ data routing switch is the core unit of the large scale baseband pool. It is capable of

switching any RRH data to any baseband processing unit for data processing. This data switch

architecture is based on the Fat-Tree architecture of DCN technology. The advantages of this

architecture include:

- Fault-tolerance and disaster-tolerance (high reliability)

- Better switch capability

- Less requirement to each switch node

The objective of Fat-Tree Network topology is to implement a non-blocking connecting data

communication network. When a computer networks use a single root node and binary tree

structure, the data communications between the computers that connect to separate trees will

go through the same root node. The switch capacity of the root node becomes the bottleneck.

The Fat-Tree topology introduces multiple nodes switch architecture with the load-balance

capability. With the benefit of two or multilayer of the switch architecture, any one high node

maintains connectivity to multiple low nodes. Then several high nodes can act as backups for

each other, and have the same capability of switch and connection. Under this structure, each

switch node has the same number of switch ports, and maintains the same required

transmission bandwidth. Therefore reduces switch capability requirement for each node. There

is at least one connection between any lower processing node and other processing node. If

one connection is out of service, redundant connections can play a backup role, which results in

a highly fault tolerant networks. As shown in the following figure:

Fig. 22 Multilayer Rout/Switch Architecture


Current commercial BBU equipment primarily used stack of baseband processing units, plus a

backplane with switching capability. It switches the RRH baseband IQ data to a specified

baseband processing unit, thereby creating a pre-planned processing capability of baseband

pool. The limitation of this approach is the amount of data flow from the interconnection

between any two equipments is limited by the capability of the backplane of single equipment.

So today‟s design can only support connection between 2 sets of equipment. Consequently

upgrading a single equipment capacity by adding more baseband process units will demand

higher switch capability of the backplanes. To combat this limitation, China Mobile Research

Institute proposed to apply the Fat-Tree structure into existing wireless BBU equipment.

Without significant changes to the existing equipment, the proposal adds a set of high layer

switch unit to form Fat-Tree Topology to gain higher switch and baseband pool processing

capacities. Similar to how the Computer network works, at this network structure, each

baseband processing Board, through the high layer network, can transfer its data to other

baseband board that is in lower utilization state. Furthermore having several redundancy

boards in the baseband pool will increase redundancy, and achieve real-time protection, thus

improving the reliability of the equipment.

However, contrasting to the computer network, IQ data routing switch has additional

characteristics.

First of all, Baseband signals require real time processing, and bound by its frame structure of

GSM/TD-SCDMA/TD-LTE protocols. Each frame has strict timing requirements. IQ data routing

switch cannot send a data packet belonging to a single carrier, over different connections to the

receiver. Otherwise it will require the receiver to rearrange the received data packet, which will

generate additional delay. The End-to-end transmission cannot be routed multiple times,

which .causes delay and jitter at the received end. China Mobile Research Institute has

proposed a Pre-distribution Routing technology to solve this problem. Its principle is to pre-

allocate resource before connection is established, making each switching node setting aside

adequate resources and identifying of the next routing port.

Secondly, IQ data transmission requires relatively large bandwidth, it is important to consider

transmission path load balancing, otherwise it could easily cause the route blockage during

overload. Therefore China Mobile Research institute has proposed the Load Balanced

technology. The principle is that: for a routing node receiving a data flow, the data flow with

the source address of Src, the object address of Dst, the flow (each data spread sent of is 1 or

multiple carriers) data numbered the Num, routing node finds the routing table based on Dst. If

the routing table includes multiple suitable “next jumps”, the routing node will generate a

random number according to (Src, Dst, Num), then determining the address of next Jump

based on the random number. This has resulted in path selection of randomization. With the

Path selection of randomization, even if the Src and Dst are same, the difference of the carrier

number (Num) will generate different path/route, so as to achieve the load balancing.

Distributed Architecture

In addition to IQ data routing, we need to consider implementation of resource management,

signal processing functions and so on, for a large scale baseband pool. China Mobile Research

Institute has introduced the Distribute Architecture. Use ZTE equipment as an example, a single

baseband processor BBU module can handle the Iub interface signaling and servicing

processing, based on the largest capacity in a network with 108 carriers. A distributed

framework can solve the problem of large scale processing, retain service processing unit for

each box. At the same time, a separate Ethernet switch handles dynamic resource management.

Each box has separate and independent Iub ports; it logically becomes independent network

elements of NodeB. In addition, one extra master network element manages entire resource of


the rack, and controls redistribution of individual physical resources. This approach is simple to

implement, adding a box means gaining one more independent NoteB network element,

without any impact to other network elements. Also, when a baseband processing unit fails, the

failed unit, under the master redistribution mechanism, can redistribute its original signaling

information to other box over the Ethernet.

6.3 C-RAN prototype based on General Purpose Processor

China Mobile, in collaboration with IBM, ZTE, Huawei, Intel, Datang Mobile, France Telecom

Beijing Research Center, Beijing University of Post and Telecom, China Science Institute, jointly

developed the C-RAN prototype supporting multiple air interfaces, entirely using platform based

on general purpose processor. The prototypes supporting GSM and TD-SCDMA have

successfully completed interoperability with commercial end user devices, while the TD-LTE

version has gone through testing with UE simulator. The prototypes have proved the feasibility

of implement GSM/TD-SCDMA/TD-LTE physical layer signal processing on general purpose

processor based platform, and a step closer to achieve greater software implementation and

upgrade flexibility.

The following sections will describe hardware and software architecture of the prototype.

As shown below, the PCI Express interface is connected to CPRI/ir interface converter, which

then carries GSM/TD-SCDMA/TD-LTE signals to commercial RRHs. IQ samples of all three

standards are processed by the commercial server in real time.

Fig. 23: IT Server Platform Topology

The C-RAN proof of concept focuses on baseband processing feasibility on IT server, therefore,

the software develop does not cover any core network functions. The baseband processing

software is developed on Linux, and has implemented Layer 1, 2 and 3 on GSM and TD-SCDMA,

and Layer 1 processing on TD-LTE, with plan to add MAC scheduling in the near future. As a

result, the system currently only supports single UE. In the future, the TD-LTE system will

support MAC, L2, L3, LTE-A features like CoMP, and completes interoperability with commercial

devices.

Signal processing carries stringent real time requirements which pose challenges to the IT

servers. GSM protocol requires each frame being processed within 40ms; TD-SCDMA frame is


5ms, while TD-LTE protocol requires every frame has to be completely processed within 1ms.

Typical IT operating system is not designed to meet telecom grade real time requirements,

therefore subframe scheduling delay, resource management are not typically guaranteed to

complete fewer than 1ms. In addition, IT platform generally lacks the stringent timing required

by base station. Lastly, traditional signal processing algorithm is typically designed to be

implemented on ASIC, FPGA and DSP. Therefore, many believe that IT server is not capable of

handling complex signal processing such those of LTE.

However, the C-RAN trial has so far proved that IT server can meet the aforementioned

challenges with technology innovations. First step is to expand the real time capability on IT

server to meet the subframe processing timing and accuracy demand. In addition, by adding

hard real time and synchronization on the CPRI/Ir interface card, we can separate the RRH

„hard real time‟ CPRI/Ir functions from the IT signal processing tasks which only require soft

real time.

Finally, significant effort had been spent to optimize LTE algorithm on general purpose processor,

fully utilizing every available instruction set and memory to the maximum advantages,

therefore significantly increases the CPU processing efficiency. We were able to implement 3GPP

release 8 TD-LTE physical layer entirely on software running on general purpose processor and

meeting all the timing and delay benchmarks. The TD-LTE implementation parameters are:

20Mhz bandwidth, 2x2 MIMO downlink, 1x2 SIMO uplink, 64QAM/15QAM/QPSK modulation,

Turbo decoder with adaptive early termination. Under peak throughput, every subframe was

being processed under 1ms TTI, meeting the most stringent HARQ processing latency

requirements in TD-LTE. As expected, GSM and TD-SCDMA processing met the timing

requirements with flying colors.

Based on trial results to date, we can conclude that CPU is capable to process baseband signal

processing work load and associated real time requirements. Cycle counts of certain modules

take up higher proportion of the overall processing time, such as turbo decoder, convolution

decoding, FFT processing etc. By introducing co-processing of such tasks, we can expect to

increase overall efficiency by 5 times or higher. In the not too distant future, general purpose

CPU implementing BBU functions, combining with DSN, will be the foundation of an open

platform that serves a large scale dynamic baseband pool, evolving into a virtualized, cloud

computing C-RAN solution.


7 Conclusions

With the arrival of the mobile Internet era, today‟s RAN architecture is facing more and more

challenges that the mobile operators need to solve: mobile data flow increase drastically caused

by the popularization of smart terminals, very hard to improve spectrum efficiency, lack of

flexibility to multi-standard, dynamic network load because of “tides effect” and expensive to

provide ever increasing internet service to end users. Mobile operators must consider the

evolution of the RAN to a high efficient and lost cost architecture.

C-RAN is a promising solution to the challenges mentioned above. By using new technologies,

we can change the network construction and deployment ways, fundamentally change the cost

structure of mobile operators, and provide more flexible and efficient services to end users.

With the distributed RRH and centralized BBU architecture, advanced multipoint

transmission/reception technology, SDR with multi-standard support, virtualization technology

on general purpose processor, more efficient way of dealing with the tides effect and service on

the edge of the RAN, C-RAN will be able to provide today‟s mobile operator with a competitive

infrastructure to keep profitable growth in the dynamic market environment.

We‟d like to invite all the mobile operators, the telecom equipment vendors, the traditional IT

system vendors, and industry/academic research institutes who are concerned on the future

evolution of the RAN to devote their intelligence and resources in the research of C-RAN to

make it a reality.


8 Acknowledgement

We would like to thank IBM China Research Lab, Intel Cooperation and Institute of Computing Technology, Chinese Academy of Sciences for their valuable contribution to this white paper.


9 Terms and Definitions

This section provides the terms and definitions for this document.

3GPP 3rd Generation Partnership Project

AIS Alarm Indication Signal

ASIC Application Specific Integrated Circuit

ARPU Average Revenue Per User

BBU Base Band Unit

BS Base Station

CAGR Compound Annual Growth Rate

CAPEX Capital Expenditure

CBF Coordinated Beam-Forming

CDN Content Distribution Network

CoMP Cooperative Multi-point processing

C-RAN Centralized, Cooperative, Cloud RAN

CSI Channel State Information

CT/CR Cooperative Transmission/Reception

DPI Deep Packet Inspection

DSP Digital Signal Processing

DSN Distributed Service Network

eNB Evolved Node B

FEC Forward Error Correction

FTTX Fiber To The X

FPGA Field Programmable Gate Array

GGSN Gateway GPRS Support Node

GPP General Purpose Processors

GSM Global System for Mobile Communications

HW/SW Hardware/Software

ICI Inter-cell Interference

IQ In-phase/Quadrature-phase)

I/O Input/Output

JP Joint Processing

LTE Long Term Evolution

LTE-A Long Term Evolution - Advanced

MAC Media Access Control

MIMO Multiple Input Multiple Output

MNC Mobile Network Controller

OBRI Open BBU RRH Interface

OFDM Orthogonal Frequency Division Multiplexing

OPEX Operating Expenditure


OTN Optical Transmission Net

O&M Operations and Maintenance

P2P Peer to Peer

PA Power Amplifier

PHY Physical Layer

Pon Passive Optical Network

QoS Quality of Service

RAN Radio Access Network

RF Radio Frequency

RNC Radio Network Controller

RRH Remote Radio Head

RRM Radio Resource Management

SDR Software defined Radio

SFP Small Form-factor Pluggable

SGSN Serving GPRS Supporting Node

TCO Total Cost of Ownership

TDD Time Division Dual

TD-SCDMA Time Division-Synchronous Code Division Multiple Access

TEM Telecom Equipment Manufacturer

TP Transmission Point

UE User Equipment

UL/DL Uplink/Downlink

UMTS Universal Mobile Telecommunications System

UniPon Unified Passive Optical Network

VNI Visual Networking Index

VoIP Voice over IP

WCDMA Wideband Code Division Multiple Access

WDM wavelength Division Multiplexing

XENPAK 10 Gigabit Ethernet Transceiver Package

XFP 10-Gigabit small Form-factor Pluggable


10 Reference

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[10] Rajesh Gadiyar, John Mangan, “Using Intel Architecture for implementing SDR in Wireless Basesations”, SDRForum, SDR09‟.

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china mobile cran white paper

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